Digbeth Vita

A trackball mouse is a brilliant alternative to the standard computer mouse that is used for a variety of issues such as arthritis, RSI-related injuries, and dyspraxia.
Ergonomic Trends has a short video demonstrating the use of a trackball mouse as well as an article on them - this link leads to the article.
Below is the trackball mouse demonstration video mentioned before:

Another option for controlling your system is to use an eye tracker, which allows you to use your eyes as a cursor to navigate your system. A well-reputed eyetracker is the Tobii Dynavox.
Below is a demonstration video of the eye tracker.